Aspects of the present invention are directed to multilevel pulse position modulation for efficient encoding of information into servo patterns.
Timing-based servo (TBS) is a technology developed for linear tape drives. In TBS systems, recorded servo patterns consist of transitions or bursts with two different azimuthal slopes, and a head lateral (y)-position is derived from the relative timing of pulses generated by a servo reader reading the pattern.
TBS patterns also allow the encoding of additional longitudinal position (LPOS) information without affecting the generation of the transversal position error signal (PES). This is obtained by pulse-position modulation (PPM) of dibits corresponding to the second and fourth stripes in certain bursts (e.g., bursts A and B of FIG. 1). Currently, systems only encode one bit of information per servo frame using binary PPM, which uses two position shift levels. The modulation depth for binary PPM in LTO is 0.25 μm.
Optimum detection of the servo patterns is achieved by a synchronous servo channel employing a matched-filter interpolator/correlator, which ensures that optimum filtering of the servo reader signal is performed not only at constant tape velocity, but also during tape acceleration and deceleration. A synchronous servo channel thus guarantees the optimum processing of a servo signal for the generation of y-estimates, velocity estimates, and LPOS information.
Optimization of the pattern geometry is of high importance in order to achieve the maximum overall system performance, including the minimization of the standard deviation of PES, the maximization of the parameter update rate to increase the control system bandwidth, and the optimization of the rate of information embedded in the modulated dibits. The servo pattern in LTO technology is mainly designed such that it enables a reliable estimation of the head y-position and the tape velocity. Furthermore, the operating point for the servo channel has to be chosen such that the signal-to-noise ratio (SNR) of the servo readback signals allows the y-position and velocity parameter estimation with sufficient accuracy for proper track-following operation. This typically requires a servo channel SNR of 20 dB or better. On the other hand, reliable detection of LPOS symbols using binary PPM requires much lower SNR.